1. Field of the Invention
The present invention relates to an apparatus and method for detecting stain on a paper sheet, in which a paper sheet which is being transported is irradiated with light from a light source, and reflected light or transmitted light thereof is received by a light receiving sensor, to determine a stain on the paper-like sheet based on an output signal for each pixel of the light receiving sensor.
2. Description of the Related Art
Conventionally, it has been required that, at the time of checking the fitness of printed matter of paper sheets such as bills, securities, and tickets, if graffiti such as a character, sign, or mark is on the paper sheet, the paper sheet is handled as an unfit paper and the paper sheet is not re-circulated in the market.
As a device that detects stains on the paper sheet, for example, a paper-sheet stain determining apparatus that detects a stain by converting reflected light or transmitted light from a sheet to an electric signal, binarizing the electric signal, adding the binarized signals (the number of pixels), and comparing the added signals with a reference value is disclosed in Japanese Patent Application Laid-open No. S63-182798.
In Japanese Patent Application Laid-open No. S63-182798, however, to detect a fine graffiti line drawn on a paper sheet with a pencil or the like as a stain, high-resolution image data having a large data size is required. Therefore, a large capacity memory for storing such image data is necessary and consequently its circuit size becomes larger. As a result, the cost of the paper-sheet stain determining apparatus increases. For example, to detect a fine graffiti line of 0.5 mm width, high-resolution image data of 100 dpi (dots per inch)×100 dpi (size of one pixel is 0.25 mm×0.25 mm) is required.
In this connection, an operation needs to be performed using image data having a large data size. Therefore, there is another problem that the processing time at the time of performing such an operation by a central processing unit (CPU) becomes greater. If a CPU having high processing speed is used, although the processing time becomes short, the cost of the CPU becomes more expensive and the circuit size becomes much larger. As a result, the cost of the paper-sheet stain determining apparatus increases as well as using the memory described above.
Therefore, it is an important issue to realize stain detection without enlarging the circuit size, when a fine graffiti line drawn with a pencil or the like is to be detected.